The present invention relates to an ion trap mass spectrometer which can be used for liquid chromatograph mass spectrometry (LCMS) and gas chromatograph mass spectrometry (GCMS).
FIG. 1 shows a liquid chromatograph mass spectrometer (LCMS) using a conventional ion trap mass spectrometer. Respective components temporally separated by a liquid chromatograph (LC) 11 are ionized at an ionization section 12, and continuously introduced to an ion trap mass spectrometer 13.
The ion trap mass spectrometer 13 includes one annular ring electrode 14 having an inner surface in one revolution form of hyperboloid, and a pair of end cap electrodes 15 and 16, which have inner surfaces in two revolution forms of hyperboloid and are disposed to face each other with the ring electrode 14 therebetween. By applying radio-frequency (RF) alternating voltage between the ring electrode 14 and the end cap electrodes 15 and 16, a quadrupole electric field is formed in a space surrounded by the electrodes 14, 15 and 16 (hereinafter referred to as xe2x80x9can ion trap spacexe2x80x9d).
Ions introduced from a through hole in one end cap electrode 15 are once captured by the quadrupole electric field in the ion trap space. In this condition, an alternating voltage (auxiliary alternating voltage) having a specific frequency is applied between the end cap electrodes 15 and 16, so that only ions having a specific mass number (mass/charge) corresponding to the specific frequency are resonated and oscillated inside the electric field in the ion trap space, to thereby exclude the ions from the ion trap space. If the voltage applied to the end cap electrodes 15 and 16 is adequately set and a collision gas is introduced into the ion trap space, the ions having the specific mass number can be excited and dissociated.
This method is used for an MS/MS analysis. The ion trap device structured as described above may be used by itself as the mass spectrometer, or a measurement with high mass resolution may be conducted by introducing the ions ejected from the ion trap space into another mass spectrometer (for example, time-of-flight mass spectrometer, that is, TOF).
In case the ions are introduced into the ion trap mass spectrometer 13, an amount of introduced ions is greatly effected by the RF voltage applied to the ring electrode 14 of the ion trap mass spectrometer 13. For example, in case the phase of the RF voltage is a positive potential, positive ions which reach the ion trap mass spectrometer 13 are bounced back therefrom, so that the positive ions can not enter into the ion trap space. Also, in case the phase is a negative potential, the positive ions are excessively accelerated, and collide with the end cap electrode 16 in the exit side to disappear. In the limited intermediate phase between the positive potential and the negative potential, only a part of the ions which reaches the entrance of the ion trap mass spectrometer 13 can be introduced into the ion trap space. A range of the phase in which the ions are properly introduced into the ion trap space as described above is several percent of the total phases, and a large number of ions are is not provided to the analysis and discarded.
Also, a trapping potential of the ion trap is inversely proportional to the mass number of the ions. In order to increase the trap efficiency of the ions, kinetic energy of the ions is required to be substantially the same as the confinement potential. However, even if such a relationship is made in the specific ion, a potential necessary for confining or trapping is high for the ions having a lower mass number than that of the specific ion described above, and on the contrary, a potential necessary for confinement or trapping is low for the ions having a higher mass number than that of the specific ion, resulting in that the trap efficiency is decreased in both cases. Namely, the trap efficiency of the ion trap has a large mass dependency.
Accordingly, an object of the invention is to provide an ion trap mass spectrometer, which can carry out a measurement with much higher sensitivity by introducing much more ions into the ion trap space, to thereby solve the aforementioned problems.
Further objects and advantages of the invention will be apparent from the following description of the invention.
To achieve the aforementioned object, the present invention provides an ion trap mass spectrometer, which comprises an ion supply source; an ion trap; an ion storing section which is disposed between the ion supply source and the ion trap and accumulates the ions at an exit side by a radio-frequency electric field with an axial electric potential inclined from an entrance side to the exit side of the ion storing section; an entrance gate electrode disposed between the ion supply source and the ion storing section; and an exit gate electrode disposed between the ion storing section and the ion trap.
The ion trap mass spectrometer may include a control section for controlling the entrance gate electrode, the exit gate electrode, the ion trap, and the ion storing section. According to the control by the control section, the entrance gate electrode is opened while the exit gate electrode is closed to introduce the ions into the ion storing section. After a first predetermined period of time, the entrance gate electrode is closed to accumulate the ions near the exit side of the ion storing section. After a second predetermined period of time, the exit gate electrode is opened to thereby emit a bunch of ions. A bunch of ions is introduced into the ion trap with the ring electrode voltage being cut off. When the maximum amount of ions stays inside the ion trap, the ring electrode voltage is suddenly applied. Thus, the ions can be trapped efficiently.